Angle of Arrival (“AOA”) is a technique employed to determine the propagation direction of a radio-frequency (“RF”) signal wave incident on a multi-element antenna array by measuring the Time Difference of Arrival (“TDOA”) of the signal at individual elements of the array. This TDOA measurement is made by measuring the difference in received phase at each element in the antenna array, and converting the measured difference to an AOA measurement. AOA technology is used for locating signal emitters such as cell phones or military transmitters, for example, to cue other precision tracking systems such as a search and track radar.
AOA measurements are typically made by measuring the phase difference of a received emitter signal between pairs of antennas in an interferometer array. Interferometer arrays are often mounted on vehicles such as ships or aircraft, and multiple arrays may be employed, e.g., to measure both elevation and azimuth angles. A significant source of AOA error can arise from interferometer ambiguities due to large systemic biases created under multipath conditions when two or more correlated signals are received simultaneously by an interferometer array. A local multipath condition such as a reflection from an aircraft wing, ship structure or from the ocean surface can significantly alter the measured phase perceived by a vehicle-mounted interferometer array (e.g., such as a pod-mounted aircraft interferometer array). In essence, the interferometer array will measure the vector sum of the direct path and the reflected path. The reflected contribution represents an aberration to the interferometer phase measurements that may ultimately make a large portion of the field of view unusable due to AOA ambiguity failures.
The application of AOA calibration terms generally requires unambiguous knowledge of the approximate origin of a signal in direction of arrival within the field of view so that the proper correction can be applied. Therefore, AOA ambiguities become problematic in the identification of proper calibration terms under multipath conditions since estimates made under these circumstances point to inappropriate correction terms.